1. Field of the Invention
The present invention relates to an optical radiation sensor device. More specifically, the present invention relates that an optical radiation sensor device with improved resistance to damage from the radiation which it is employed to measure.
2. Description of the Prior Art
Optical radiation sensors are known and find widespread use in a number of applications. One of the principal applications of optical radiation sensors is in the field of ultraviolet radiation fluid disinfection systems.
It is known that irradiation of water with ultraviolet light will disinfect the water by inactivation of microorganisms in the water, provided the irradiance and exposure duration are above a minimum "dose" level (often measured in units of microWatt seconds per square centimeter). Ultraviolet water disinfection units, such as those commercially available from Trojan Technologies Inc. under the tradename UV600, employ this principle to disinfect water for human consumption. Generally, water to be disinfected passes through a pressurized stainless steel cylinder which is flooded with ultraviolet radiation. Large scale municipal wastewater treatment equipment, such as that commercially available from Trojan Technologies Inc. under the tradename UV3000, employ this same principle to disinfect treated wastewater. Specifically, ultraviolet radiation emitting lamps are submerged in an open channel wherein the treated wastewater is exposed to radiation as it flows past the lamps. For further disclosure of fluid disinfection systems employing ultraviolet radiation see U.S. Pat. Nos. 4,482,809; 4,872,980 5,006,244; and 5,418,370, the contents of each of which is incorporated herein by reference.
In many applications it is desirable to monitor the level of ultraviolet radiation present within the water under treatment. In this way, it is possible to assess, on a continuous or semi-continuous basis, the level of ultraviolet radiation, and thus, the overall effectiveness and efficiency of the disinfection process.
It is known in the an to monitor the ultraviolet radiation level by deploying one or more passive sensor devices near the operating lamps in specific locations and orientations. These passive sensor devices may be photodiodes, photoresistors, or other devices that respond to the impingement of the particular radiation wavelength or a range of radiation wavelengths of interest by producing a repeatable signal level (in volts or amperes) on output leads.
Generally, the measurement of relatively high intensity radiation can be problematic due to the harshness of the environment in which the sensor device must be disposed and used. For instance, in an environment with a relatively high ultraviolet radiation level, an unprotected photodiode is subject to the immediate onset of rapid and irreversible degradation of the sensor device. Moreover the rate of degradation increases at increasing radiation levels. Degradation of the sensing device is characterised by reduced accuracy of the sensor device output signals, and eventually by outright failure of the sensor device.
One solution proposed to reduce degradation of the sensor device is to relocate the sensor device further from the radiation source. Practically, this creates other problems relating to geometry or size constraints of the sensor device and/or the fluid disinfection system.
Another solution proposed to reduce degradation of the sensor device in harsh environments involves the use of special filters and coatings for the radiation sensing device. One example involves the use of a filter glass that is placed between the radiation source and the sensor device in order to attenuate the radiation or to remove unwanted wavelengths, or both. In this case, the filter glass is separate from the sensor device. Unfortunately, the filter glass may, itself, be subject to degradation because of the radiation present or other influences associated with the environment which have a degrading effect. Further, the filtering effect provided may change over time, reducing the accuracy of the sensed values. Another example involves the precipitation of a phosphorus coating onto the face of the sensor device to provide a wavelength conversion effect thereby minimizing degradation of the sensor. Unfortunately, the phosphorous coating itself is subject to degradation along with the sensor device.
It would be desirable to have an optical radiation sensor device which has an improved resistance to the degradation that results from prolonged use in an ultraviolet radiation environment.